An increased demand of fuel for power plants and for other types of industries, particularly in view of the high cost of oil, as well as the short supply of fuel oil, has created a situation in which coal has become most valuable and cheapest fuel available. An important factor which influenced the change from coal to oil and gas in previous years has been the cleaner burning of the oil and gas which reduced the emission of pollutants to the atmosphere.
Among the most damaging of the air pollutants are the sulfur oxides resulting from the burning of sulfur-containing fuels. Natural ambient air quality standards and emission standards can generally be met in the case of the burning of coal only if the coal contains less than 1% of sulfur.
The sulfur in coal occurs mainly in organic and inorganic forms, only a small amount of elemental sulfur sometimes being included. A substantial amount of the inorganic sulfur (pyritic sulfur) may be removed from sulfur by cleaning the coal with water and surface active agents which help to separate the coal from the heavier sulfur-containing minerals. However, these known processes result in substantially all of the organic sulfur remaining in the coal subjected to such flotation treatment, as well as a part of the pyritic sulfur also remaining therein.